1. Field of the Invention
This invention relates, in general, to protection of integrated circuits and more particularly to protection and recovery from latch-up of integrated circuits.
2. Description of Background
Typically, latch-up and subsequent burn-out of some junction isolated integrated circuits (IC) may be caused by either internal or external sources. Voltage transients of the power supply line or propagation of spurious signals such as negative voltage spikes into an input port of the IC are examples of internal sources. These internal sources trigger the IC into a latch-up condition characterized by a high conduction current. When this occurs, burn-out of the IC is possible should there be no means to limit the amount of current through the IC. Presently, latch-up is prevented through the use of circuitry in the power supply line and the input ports. This tends to delay the signal processing of the IC and increase the cost of the overall device using the IC.
An example of an external source causing latch-up would be transient ionizing radiation from a nuclear explosion or the like. This radiation would generate a photo-current which would trigger a latch-up prone IC into the high conduction state. Currently there is no practical way such as spot-shielding to prevent nuclear ionizing radiation from reaching the IC and triggering the IC into latch-up.
The current method generally used to prevent latch-up is to limit the maximum IC current with a resistor to a value below the holding current of the IC. The disadvantage of this method is the need to know in advance the holding current in order to properly select a resistor that would limit the current below the holding current. In addition, the voltage drop across the resistor may be greater than the allowable limits.
One method currently used to prevent latch-up induced by transient ionizing radiation is through the use of an inductance-capacitance network in the supply line. This network is designed to slow down the increase in current flowing into the affected IC. The time constant of the network is to be significantly long relative to charge storage and recombination time of the affected IC. This method requires knowledge of the latch-up on-set current and the charge storage and recombination time of the IC.
Presently should latch-up occur the only way to delatch an IC is to shut down the circuit, which generally means shutting down the entire system, and then turning the system on again. This can cause intolerable delays of time and prevent use of any apparatus using a latched-up IC.